- The smooth muscles (blood vessels, intestines, uterus) (contracted involuntarily).
Striated muscle consists of numerous elongated fibers.
Muscle fiber is a multinucleated cell, covered by elastic coat – sarcolemma. The muscle fiber includes motor nerves, which transmit nerve impulses to it, causing contraction. There are filamentous formations – myofibrils – in the semiliquid sarcoplasm along the length of the fibers. Sarcomereis a repeated element of myofibrils, limited by Z-line (Fig. 9). In the middle of the sarcomere there is the A disk (dark in phase-contrast microscope); in the centre of which there is M-line, visible by electron microscopy. H-zone occupies the central part of the A-disk. I-disks are light in a phase-contrast microscope, and each is divided into equal halves of Z-line. In the A-discs there are thick myosin and thin actin filaments. Thin filaments begin at Z-lines, and then they pass through the I-disk and are interrupted in the region of H-zone. Electron microscopy showed that thick filaments are arranged in a hexagon shape and pass through the A-disk. The thin filaments are between the thick ones. During the contraction of muscle, the I-discs virtually disappear, and the region of overlap between thin and thick filaments increases.
Fig. 9. The structure of the muscle fiber
Sarcoplasmic reticulum is the intracellular membrane system of interconnected flattened vesicles and tubules; it surrounds the sarcomeres of myofibrils. There are proteins on the inside membrane which are capable of binding calcium ions.
9.1. Chemical composition of muscle tissue
The muscle tissue consists of 72-80% of water; 20-28% is dry residue (proteins, carbohydrates, lipids, non-protein nitrogen-containing substances, etc.).
Muscle proteins are sarcoplasmic (35% of muscle protein), myofibrillar (45%) and stromal proteins (20%).
Sarcoplasmic proteins are soluble in salt solutions with low ionic strength. It is a mixture of proteins with properties of globulins; proteins with enzymatic activity (glycolytic enzymes); myoglobin; enzymes of tissue respiration, oxidative phosphorylation, nitrogen and lipid metabolism; parvalbumins linking calcium ions.
Myofibrillar proteins are soluble in salt solutions with high ionic strength and are closely associated with the contractile function of muscles. They are myosin, actin, actomyosin, regulatory proteins (tropomyosin, troponin, α- and β-actinin).
Myosin is 50% dry mass of myofibrils. It is elongated molecule which consists of two subunits: two heavy polypeptide chains (Mr = 210 000) and a few of light (Mr = 20,000).
Myosin heavy chains are long a-helix. It is the "tail" of the molecule. The end of the heavy chain (together with the light one) makes up the "head" of the molecule (globule), which is combined with actin. Light chains of "head" of the myosin molecules are involved in the manifestation of the ATPase activity of myosin. Myosin is inhibited by Mg2+ ions and is not inhibited by high concentrations of ATP. Sarcomere thick filaments are the connections of many myosin molecules definitely oriented in the space.
Actin (20% dry mass of myofibrils) is presented in two forms: globular (G-actin) and fibrillar (F-actin). G-actin is polypeptide chain (globule). Polymer of G-actin is F-actin which molecule is similar to the two strands of beads twisted around one another.
Actomyosin is a complex of myosin with F-actin. It has ATPase activity. Actomyosin ATPase is activated by magnesium cations, and inhibited by high concentrations of ATP.
Tropomyosin consists of two α-helices and has the form of stick (4-7% of the proteins of myofibrils).
Troponin (Tn) – is a globular protein (2% of the proteins of myofibrils), has subunit which inhibits ATPase activity (Tn-I), calcium-binding (Tn-C) and tropomyosin-binding (Tn-T) subunits.
Troponin complex with tropomyosin is called native tropomyosin. It attaches to actin and allows sensitivity of actomyosin to calcium ions.
Stroma proteinsare collagen and elastin.
Non-protein nitrogen-containing substancesarenucleotides, adeninylic primarily (ATP, ADP, AMP). Creatine and creatine phosphate are involved in processes of muscle contraction. Creatine is synthesized in the liver, with bloodstream is transferred to the muscle tissue and there it is phosphorylated. Creatine phosphate is formed. Carnosine and anserin are imidazole containing dipeptides that are specific to skeletal muscles of vertebrates. They increase the amplitude of muscle contraction during fatigue. Amino acids are glutamic acid and glutamine.
Phosphoglycerides – phosphatidylcholine, phosphatidylserine, phosphatidylethanolamine, and others are part of the cell membranes of muscle tissue, they are substrates of tissue respiration.
Products of nitrogen metabolism – urea, uric acid, adenine, guanine, xanthine, hypoxanthine – occur in the muscles in small quantities.